Implement and document a virtual env for the webscraper project

[philion]

Add a requirements.txt to capture dependencies. Update README with setup instructions for the venv.

To save:

python -m venv .venv
. .venv/bin/activate
pip install -U pip           # update pip
pip freeze > requirements.txt

To create venv and install:

python -m venv .venv
. .venv/bin/activate
pip install -r requirements.txt

[philion]

Complete.